This invention relates to the manufacture of hydrodynamic seals. More particularly, the invention relates to an improved process for the formation of hydrodynamic seals from polytetrafluoroethylene (PTFE) and the like.
Hydrodynamic seals are widely used as sealing elements for preventing the passage of fluid along a rotating shaft. Hydrodynamic seals are generally annular in shape, having a frusto-conical or truncated central portion adapted to bear against the rotating shaft. The truncated portion includes a spiral groove along its inner surface that functions to facilitate the return of any fluid, e.g. oil, which may be present on the shaft to the oil side of the seal.
Because of its ability to withstand high temperature operation, PTFE is generally acknowledged as a highly desirable material for use in the formation of hydrodynamic seals. However, due to difficulty of molding materials such as PTFE, into the desired shape for sealing elements, alternative methods of manufacture are employed. The methods heretofore proposed require expensive and cumbersome machinery to form the truncated portion and/or the inner spiral grooves. Those requirements result in increasing the cost of the manufacture of such seals.
In accordance with one contemporary technique for the manufacture of PTFE hydrodynamic seals, a spiral groove is machined on the face of the PTFE billet prior to slicing an annular section from the rotating billet. After slicing, the resulting section is then deformed under pressure to form a truncated central portion having the spiral groove on the inner surface of the truncated portion. Exemplary of this method of fabrication is the technique set forth in U.S. Pat. No. 3,857,156 to Clark. Though such a method produces an adequate sealing element, it has the disadvantage of requiring a press-forming operation to form the truncated inside surface of the seal. Such an operation not only necessitates additional manufacturing time and purchase of expensive deforming machinery, but also introduces the possibility that the sealing element may be deformed unevenly and thus result in a defective seal.
In accordance with a second contemporary manufacturing technique, an annular section is again deformed under pressure to form a truncated portion. The spiral groove is then formed on the sealing element by continuing to press the truncated portion against a threaded surface. This press forming, or coining process also requires expensive additional machinery, such as die tooling apparatus, which also increases the potential that the sealing element may be formed defectively. Exemplary of such techniques are those disclosed in U.S. Pat. Nos. 3,929,341; 3,939,551; and 3,985,487, issued to Clark, et al.